During a soldering process, two or more parts are joined to one another with a solder. As is known in the art, and by definition, solder is an alloy or filler metal having a liquidus temperature not exceeding 450° C. (840° F.). While in contact with the parts to be joined, the solder is melted and flows into the joint by capillary action. Upon cooling the molten solder, a permanent joint is formed between the parts. One common application of solders is in the electronics industry where solders are used to join electrical components. Solder may be supplied by a multicomponent paste that is deposited on the surface of at least one of the parts to be joined. Typically, the paste contains a flux and particles of a solder alloy. The flux is formulated to improve the soldering operation. For instance, fluxes are often formulated to remove surface contamination, like surface oxidation, from metallic parts. In that way, the flux may also improve the flow and wetting of the solder alloy across the metallic surface. In a soldering operation, once the solder paste is dispensed onto the surface, the solder paste is heated to a temperature where the flux prepares the surface. Upon further heating, the solder particles melt and flow across the prepared surface and into the joint, and following cooling, the solder forms the permanent joint.
Even though current solder pastes facilitate forming consistent, quality solder joints, there are difficulties with their use due to the pastes being temperature sensitive, in the sense that the pastes degrade or deteriorate when exposed to temperatures at or above room temperature prior to soldering. Specifically, with regard to temperature sensitivity, if the solder paste is exposed for prolonged periods at temperatures even as high as room temperature, it may gradually lose its beneficial attributes. Moreover, exposing the solder paste to greater temperatures hastens its deterioration. Consequently, current pastes are viewed as having a limited shelf life at and above room temperatures.
One solution to extend the shelf life of current pastes is to refrigerate them up to and including the time that the solder is deposited onto the parts to be joined. Refrigeration arrests or reduces chemical reactions from taking place within the paste and prevents the solder from separating from the flux prior to depositing. Typical refrigeration temperatures include temperatures generated by commercially available refrigeration or air conditioning equipment, such temperatures being generally below 10° C. However, refrigeration has significant drawbacks. Most notably, there are high capital and operational costs associated with using refrigeration equipment. Furthermore, more often than not, the nature of the manufacturing environment precludes refrigeration as a means for preventing degradation of the solder paste due to the elevated temperatures inherent in these environments or due to other factors including, for example, floor space within the manufacturing plant. Thus, solder pastes are not used in these environments because their beneficial attributes are limited or destroyed prior to the time that these benefits may be fully realized and because of the lack of a cost effective solution to these problems.
Another problem that limits or renders difficult the use of current solder pastes is that the solder paste remains soft or pasty following deposition. For example, problems arise when the solder paste is pre-deposited on the part but the actual soldering operation is performed at a later time and/or at another manufacturing facility. In the interim period between the deposition and the soldering operations, if the part is stored and/or handled, foreign objects may contact, rub against, or become stuck in the soft solder paste. In addition to the problems encountered due to foreign material being stuck to the paste, the paste may stick to the foreign object and adhere to that surface. This type of contact depletes the original deposit of paste, and, in extreme situations, the original deposit may be significantly depleted or even completely removed from the targeted surface. Additionally, the paste may inadvertently transfer to surfaces where the paste or solder alloy is detrimental to the part. In any event, the pasty, flowable nature of the paste at around room temperature limits post-deposition operations and may add to the manufacturing costs. While refrigerating the solder paste may improve the shelf life of the solder paste, it may not result in the solder paste becoming non-pasty at these temperatures. In other words, a cold solder paste may have all of the above-mentioned inadvertent transfer issues. In addition to the cost considerations set forth above, refrigeration is not generally a solution to preserve the deposited solder pastes.
One solution that partially addresses transportation and handling problems includes immediate reflow of a dispensed paste with a subsequent application of flux to protect the reflowed solder alloy. During an initial reflow, the solder paste is fused at the predetermined, deposited locations. Following cleaning, the solder alloy is flattened by reheating the deposited, fused solder alloy and is then cooled. An outer coating of flux is applied to the flattened solder to prevent oxidation of the solder and promote later reflow of the solder. The flux-covered, reflowed solder alloy may then be transported or stored for later use. However, as noted, such a process requires that the part be heated at least one additional time which may be counterproductive since, in the case of electronic assemblies, the parts themselves may be temperature sensitive. The additional heating cycle may increase the number of component failures. Furthermore, the flux remains tacky such that a release sheet is attached to the flux to prevent contamination.
Consequently, there is a need for a solder flux and solder material that addresses the aforementioned problems. For example, what is needed is a flux and/or a solder material that is non-pasty at or near room temperature and at temperatures above these temperatures including normal manufacturing, storage, and shipping temperatures. Furthermore, what is needed is a solder flux and solder material that may be heated prior to deposition, that does not degrade at the deposition temperature, and that may be cooled and reheated without degrading.